DIYbanter - View Single Post

rote:

Now it just fired up again. It went off on thermal no doubt. I
still
have to do something, I can't let this thing run unnattended. Thing
that bothers me is how soon it went off on thermal, maybe the
driven
shaft bearings are locking up ?

One respondant mentioned duty cycle. I kind like that, don't bother
rectifying anything. Just use an SCR. I could trigger it from a
single
stage flip flop then to get the 30 Hertz. Heck I could even do 20
Hertz
with flip flops, and just cut the duty cycle accordingly.

Okay I'm new here but if anyone knows induction motors here, it seerms
to be me. If this is the same kind of fan I am thinking of that is
often used as a portable windstorm for low-budget movies, what you
are proposing to do is a good way to burn up a brand-new motor. What
noone here seems to know is that the stator core contstruction is
specifically designed for the frequency response, using a variable
frequency power supply, or a variac with such a fan is a sure way to
cause a meltdown. You do not "cog" or run an induction
motor on a "duty cycle" other than an on and off period.
Induction motors do not respond to SCR switching like a DC motor
does.

The thermal is blowing on the motor because you are overloading it.
The formula for fans is as follows:

Cu. ft. air per minute x water guage pressure (in.)
HP = ________________________________________
6,350 x % efficiency

where approx efficiency of a propeller-type fans is as low as 35%

Points about driving fans to remember:
The volume of air delivered by a fan varies directly with the fan
speed
The pressure produced by a fan varies as a square of the fan speed
The HP of a fan varies as the cube of the fan speed

To replace an 1100 rpm motor with a 1750 rpm motor will require twice
the horsepower. If this fan is pulley-driven, you need to adjust
pulley sizes to accomodate your speed requirements. a 20" fan
blade sounds like nothing more than the typical box fan.

Alternately you can try to be a bit more creative and try to repair
the fault in the motor, or even rewind it. If you can't manage this,
just get a new motor. I like the idea of using the treadmill motor,
as if you are brilliant enough to strip it's speed control as well,
you will have a fully-variable fan out of it, and treadmill motors
are rated at least 1 horsepower. Just be sure that if you get the
opportunity to strip a treadmill, that you take ALL of it's
electronics and electrical, most importantly all parts of the speed
control including the slider pot on it's console for speed
adjustment. If you can remove the flywheel from the motor (if not
set-screwed it is threaded on), do so and use a pulley that will fit.
I would suggest grinding a flat or keyway into the shaft (keep metal
dust out of a permanent magnet motor), and then being sure not to
exceed 1800 RPM on a 20" fan blade or you will have quite a mess
on your hands...

Anytime you want to replace a motor that has burnt out, always check
for binding in the driven side first, then for binding in the motor
itself. Belt pressure should ideally be at just barely above the
point where slippage occurs, but it seems lack of actual care for the
motor is what caused the burnout in the first place because either a
bearing went dry and gummed-up, or the motor got loaded up with dust
and debris and overheated as a result of the insulating layer of
crud. Try a bi-annual maintenance schedule next time.....

For this application, the ideal motor is a Permanent Split-Capacitor
motor (capacitor start/capacitor run with the same capacitor). This
is the same type of motor used in 14" or so desktop fans and is
always accompanied by a low-valued capacitor that is constantly
energized. The PSC motor is essentially a two-phase motor adapted for
single-phase, and the ideal as a fan motor. This motor is also
switchless so there is no switch to maintain. Find yourself a
japanese-made washing-machine and you will likely have a perfect
motor for it....

Any questions adress me directly or PM me